Is your the 5881 model then no it is made to only accept 6L6 type tubes.

SED =C= are awesome tubes, but very expensive. I swear by there powertubes now.

JJ are a good budget tube. JJ also make the KT66 which is a little diffrent than the regular 6L6 but it is a drop in replacement. JJ Ecc83 are great in a Marshall and I really like the new Mullard 12ax7

Asfar as your amp, I could not tell you, but it sounds like your powertubes are shot. Are any of the fuses blown?

The JCM900 originally came fitted with EL34's. Some American ones had 5881 tubes (basically 6L6's). They have a different screen resistor. If it is one of those you can't just go shoving EL34's in there.

Is your the 5881 model then no it is made to only accept 6L6 type tubes.

SED =C= are awesome tubes, but very expensive. I swear by there powertubes now.

JJ are a good budget tube. JJ also make the KT66 which is a little diffrent than the regular 6L6 but it is a drop in replacement. JJ Ecc83 are great in a Marshall and I really like the new Mullard 12ax7

Asfar as your amp, I could not tell you, but it sounds like your powertubes are shot. Are any of the fuses blown?

I'll check them out, thanks.

And yes, there are 2 fuses under "Output Valve Fuse" and both are gone.

Quote:

Originally Posted by Cathbard

If you've blown the HT fuse you've most likely lost a power tube.

The JCM900 originally came fitted with EL34's. Some American ones had 5881 tubes (basically 6L6's). They have a different screen resistor. If it is one of those you can't just go shoving EL34's in there.

You always have to check the bias when changing the power tubes. On the plus side, you can do a quick rough bias with just a multimeter. You just adjust the bias pot until you get -50Vdc at the junction between R26 and R27.
That does mean poking around inside a live amp though so don't do it if you don't know what you are doing.
Here's the schematic:http://www.drtube.com/schematics/ma...60-02-iss14.pdf

You can fix channel 2. Refer to the this as I explain it so you can follow what I am saying.

The problem with channel 2 is what I consider to be one of the greatest design flaws of all time.

See BR2 and D10? BR2 is a bridge rectifier and together with the D10 forms a clipper that limits how much signal goes to IC4b by clipping off the top. This results in that ugly fizz that is driving you away from that channel.
Without the clipper you can actually drive V1 harder and can get just as much distortion. So what that bit of circuitry does is replace tube distortion with solid state distortion. just
So what you do is get rid of the clipper and replace it wit a 100K resistor. You can just remove BR2 and put the resistor in the holes left behind so it connects C36 to ground.

Channel 1 has some clippers too (LED 1-4) but they aren't so bad. That's doing the tubescreamer type thing. As you've found yourself, it's ok. Channel 2 is the bitch that needs to be slapped.

JJ are the to go tube in this case, but I personally prefer EH for a bit more "rude" sound. I've also put in Tung-Sol and Shuguang in there for different results. The Tung-Sol are a bit more rounded, while the Shuguang were more midrangey with a dip in the lows.

Second, if there is a blown fuse then don't just replace it and keep chugging along. Fuses are there so when something in the amp malfunctions, the electric signal can be stopped before other components are damaged. Figure out what's wrong with your amp, get it fixed, then replace the fuse.